A Ditch in Time

Rick Stryker, P.E.

Rainfall and snow melt runoff are sworn enemies of the roads and pathways in camp. Erosion moves your road surface to places you don’t want roads. Spontaneous, unplanned drainage channels cut themselves into your improvements and through areas you’d rather not have them. Snow that melted at noon refreezes at sunset, causing the surface to heave and buckle over and over until it simply falls apart. If you think that fixing this kind of damage is expensive, then you’ll surely agree that fixing it more than once borders on criminal. This month, we’ll look at a couple of situations common in and around roadways, their causes, and how to fix things right — the first time.

Very few camps have underground pipe networks to convey storm water —instead, camps typically have open ditches and culverts. You’d think that by having absolutely no moving parts, ditches would be simple and that nothing could go wrong. But you’ve seen overflowing ditches and culverts, so you know that’s not the case. For a roadside ditch, the capacity to move water away from the road and release it in a “safe” place is fundamentally affected by these three attributes: shape, longitudinal slope, and covering. We’ll look at each of these in order.

Shape

The ancient Romans knew when they built the viaducts that circles or semicircles are the most efficient cross sections to move water in, so it’s no accident that drain pipes are round. Unfortunately, constructing a semicircular earth ditch isn’t a very practical exercise. The next best shape is a trapezoid. Recall from basic geometry that a trapezoid looks like a triangle with the top point cut off parallel to the bottom. For ditches, the widest part is at the top, and the narrowest is at the bottom. The sides of a ditch shouldn’t be any steeper than 2:1, meaning that for every two feet of horizontal side width, the side rises one foot vertically. This steepness allows for safe foot travel and the ditch side to be mown safely with simple equipment. The bottom should be at least twenty-four inches wide, both for safe maintenance and to allow small amounts of runoff to soak into the ground instead of running off at all. If you sketch that out, you’ll find that a ditch one foot deep is six feet wide on level ground! That may seem large, but if you think about it, it needs to carry the water that falls on half the road (at least six feet), plus all of the ditch itself. So in truth, those are the minimum dimensions for a ditch that is stable, can be maintained, and will pass runoff reliably.

Slope

As for longitudinal slopes, you know that they generally follow the grade of the road. Where the road runs up a steep hill, the ditch runs parallel and is just as steep. Where new roads are being constructed, the slope of the ditch doesn’t necessarily have to mirror the road slope, just like the slopes on buried pipes wouldn’t. For example, if the road is on generally flat ground, the ditch may need to get deeper over a distance, forcing water to flow in a particular direction. Keep in mind that the steeper the ditch, the faster the water will want to travel. We’ll see shortly how that can cause new and different troubles.

Covering

Roadside ditches can be covered (or lined) with many materials. In my September/ October 2009 column, “Guess What Runs Downhill?” we touched briefly on plantings that do a great job in reducing and even preventing erosion. Crown Vetch overseeded with seasonal rye grasses can be combined to generate quick growth (the rye) to hold the bare soil in place while the slower-growing Crown Vetch overruns the rye, establishing a thick vegetative mat. The trouble with this is that, although the mat holds the soil in place really well, it also slows the flow of water in the ditch. Its viney, web-like nature will choke most mowers, and over time the overgrown ditch will contain all vetch with no room for runoff! So having solved the erosion problem, we’ve ultimately created a new one with a ditch that doesn’t allow water to move freely.

Instead, facilities should plan on stabilizing roadside ditches with a slow-growing, hearty grass, which sets a root mat in the top few inches of the soil and tolerates mowing. Your state’s Department of Transportation (DOT) district office or county agricultural extension agent can help you choose a seed mix and fertilizer that works well in your climate and native soils. If you don’t have their contact information in your rolodex (that’s the old-time version of a Blackberry for you youngsters . . .), this is a great opportunity to meet them and let them help you out with what they do well.

Sometimes, the ditch slope is so steep that, although you can plant seed and even get it started, the first big rain washes it downhill. In these cases, you may consider temporary or permanent mechanisms, much like the rye/vetch seed plan. One temporary method uses a temporary mat to slow the water flow until the roots are established. Often made of coconut fiber or jute, these will deteriorate over several months, adding nutrients to the soil as the grass grows through and spreads beneath them. These need to be anchored to the sides and bottom of the ditch following the manufacturer’s instructions carefully. In particular, the upstream end of the mat must be buried, the mat must be unrolled smoothly, and the anchoring staples must be installed at every marked location on the mat. By the time that the ditch needs to be mown, the mat will have broken down, and it won’t interfere with your work or damage the equipment.

Some places flow so fast that a stable grass root mat can never be established — even with jute or coconut helping. You may have these sorts of situations at culvert outlets where water shoots out like a water cannon, pounding the soil and the channel. Again, as mentioned in “Guess What Runs Downhill?” these locations may require the installation of angular rocks sorted and grated for size called “riprap.” These rock beds disrupt the smooth stream of water, adding turbulence, and slowing the water to non-erosive speeds. Remember, though, that riprap is a permanent measure, and for it to work correctly, the rocks’ angular faces need to be exposed to the flowing water. Growing weeds and washed soil that fills spaces between rocks will change the way the liner performs. Regular mainte¬nance is required for a riprap ditch, but clearly, putting a mower through a field of eight-inch rocks is a fast way to trash it. Instead, a correctly applied herbicide (best used in the spring!) will work won¬ders at keeping the weeds down. Also, many professionals require installation of a non-woven filter fabric between the soil surface and the first layer of rocks. This inhibits weeds from growing through the riprap in the first place. Remember that the shape and geometry affect the capacity of the channel. In order to install riprap, you’ll need to excavate the chan¬nel to receive and accommodate the layer of rock. Don’t just dump it in the ditch and think the problem is solved. Finally, the size of the rocks for your job depends on the amount of water expected in the channel, its velocity, and the characteris¬tics of the soil beneath. Again, both your regional agricultural extension agent and the DOT folks solve this sort of problem all the time, and they’ll have graphs and charts that will apply to your area.

Culverts

And what about those culverts? Those are the pipes that cross under your road to pass water from one side to the other. Another time, we’ll talk about how to pick the right size culvert if you’re starting from scratch. For now though, let’s just restate the obvious: On the same slope, a larger pipe carries more water than a smaller one (usually), and a culvert on a steeper slope will carry more than the same size on a flatter slope (again, usually). The exceptions to those common sense rules are too complex to get into here, but know that they exist and they’re every day tools in a hydraulic designer’s bag of tricks.

So how do you go about replacing a culvert? Since we’re following a common sense approach, use your common sense! If the existing one isn’t passing water smoothly without flooding the road, either the volume of runoff has changed, or the capacity of the pipe has changed. How might the water volume have changed? Start by looking upstream to see whether the new parking lot, cabin circle, or athletic field has redirected runoff and overloaded the culvert. Have you paved the road you’re trying to drain without planning and installing conveyances for the increased runoff? If you can’t attenuate the runoff at the source, you may need to replace the pipe with a larger size. (Pay attention to the discus¬sion on “pipe cover” that follows to avoid creating new and bigger problems.)

If the pipe has filled with soil, sand, and debris, it’s important to determine the source. Is your ditch eroding, requir¬ing new or better stabilization? Is the ditch slope at the upstream end of the pipe very steep and the pipe very flat? If so, the high velocity water may be slowing down in the culvert and dropping its mineral load there, causing the clogging. Consider adding a stilling basin at the inlet end to allow the material to drop out of suspension before the water enters the pipe, or reinstalling the pipe at a steeper angle to keep the velocity up.

If the pipe’s been crushed, you should replace it. I usually specify pipe made of either reinforced concrete or high density polyethylene (HDPE). Any drainage pipe you install should be HS-20 rated, mean¬ing that it’s intended to handle traffic loads without deforming, provided that there’s enough soil between the top of the pipe and the road surface (“cover”). Both of these pipe materials have specific requirements for cover based on the construction class of the pipe, and you’ll need to consult the manufacturer’s literature for the specifics. Generally speaking though, twelve inches is the minimum cover for those. Aluminum or steel require more, often on the order of twenty-four inches.

Much of the work related to ditches and culverts is very intuitive and common-sense based. But after you’ve tried to think through the causes for the offending condition and tried several repairs without success, it’s probably time to get help. Professional designers solve these sorts of problems every day, and making the investment in their time and counsel is smart money.

Rick Stryker is a professional engineer with a particular passion for helping camps with infrastructure, planning, and regulatory issues. He can always be reached at campfc@ptd.net or (570) 828-4004.